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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada.Containers.Vectors - querying multiple elements
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Date: Mon, 2 May 2005 19:04:32 +0200
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Date: 2005-05-02T19:04:22+02:00
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On Sun, 1 May 2005 22:21:42 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:txah28f4zlnp$.1pqea609lovyn.dlg@40tude.net...
> ...
>> Now consider the following test:
>>
>>    type T is mod 1;
>>    type T_Array is array (T range <>);
>>    X : T_Array := <somehow-obtained-empty-array>;
>> begin
>>    for I in X'Range loop -- This is OK
>>       null;
>>    end loop;
>>    for I in X'First..X'Last loop -- This cannot be OK
>>       null;
>>    end loop;
>>
>> Which is in a perfect contradiction with 3.6.2(7). It could probably be
>> mended too if we had universal index types. But this is yet another story.
> 
> In Ada as currently defined, you would get Constraint_Error from the attempt
> to define the bounds. That happens with string types:
> 
>     type T is mod 1;
>     type T_Array is array (T range <>) of Character;
>     X : T_Array := ""; -- Raises Constraint_Error
> 
> There is actually an ACATS test to check this. (I remember it because one
> vendor disputed the test; it was upheld.)

So "" is not allowed for any string type which index is not a
subtype/derived type S of some base type T, such that S'First>T'First. With
a nice consequence that:

type T1 is range -2147483647..0;
type T1_Array is array (T range <>) of Character;

type T2 is range -2147483648..0;
type T2_Array is array (T range <>) of Character;

type T3 is range -2147483649..0;
type T3_Array is array (T range <>) of Character;

X : T1_Array := ""; -- Legal
Y : T2_Array := ""; -- Illegal
Z : T3_Array := ""; -- Again legal!!!!!

That's really funny. What is so special in the number -2147483648? Should
it mean that potentially no program that uses negative indices and empty
strings is portable, you never know where *a* built-in integer type may
start?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de